The present invention relates to a heat/cool control technique of performing temperature control by properly switching a heat mode of outputting a manipulated variable to a heating actuator and a cool mode of outputting a manipulated variable to a cooling actuator and, more particularly, to a limit cycle autotuning method and heat/cool control apparatus which adjust control parameters by generating a limit cycle with a constant manipulated variable amplitude.
A control technique called heat/cool control which selectively uses the heating capacity of a heater and the cooling capacity of a cooler is used for temperature control on air in the chamber of a thermostatic chamber unit, temperature control on a plastic material in an injection molding machine, and the like. The typical arrangement of an apparatus using this technique includes a mechanical unit which can heat and cool air and a fan which sends air with an arbitrary temperature at a proper airflow and sends heated or cooled air into a chamber/room as a control target.
FIG. 9 shows the arrangement of such a heat/cool control system. Reference numeral 21 denotes a thermostatic chamber; 22, a heater which heats air; 23, a cooler which cools air; 24, a temperature sensor which measures the temperature in the chamber; and 25, a heat/cool control apparatus. The heat/cool control apparatus 25 computes a manipulated variable MV on the basis of a set point (intra-chamber temperature set point) SP set by an operator and a controlled variable (intra-chamber-temperature) PV measured by the temperature sensor 24, and outputs the result to the heater 22 or cooler 23. The heating capacity of the heater 22 is increased/decreased by an SCR in accordance with the manipulated variable MV. The cooling capacity of the cooler 23 is increased/decreased by an inverter in accordance with the manipulated variable MV.
In brief, a control technique for the heat/cool control system in FIG. 9 is a temperature control technique which implements one PID control system and is of a heating/cooling switching type that operates the heater 22 in accordance with the manipulated variable MV (heat mode) if the manipulated variable MV is 0% or higher and operates the cooler 23 in accordance with the manipulated variable MV (cool mode) if the manipulated variable MV is lower than 0%, as shown in FIG. 10. As described above, the heat/cool control is a control technique which implements a single loop control system which is applied to an apparatus arrangement including heating and cooling actuators in one loop temperature control system and handles the heating and cooling actuators as a set. According to this technique, the manipulated variable MV is output to a heat-side operation end (heating actuator) in the heat mode, whereas the manipulated variable MV is output to a cool-side operation end (cooling actuator) in the cool mode.
In the above heat/cool control system, the PID parameters in the heat/cool control apparatus 25 must be switched to proper values in each of the heat mode and the cool-mode. In order to automatically obtain this proper value, an autotuning method has been proposed. This autotuning method is disclosed in, for example, Japanese Patent Laid-Open No. 5-289704. The autotuning method uses a limit cycle scheme. First of all, PID parameters on the heat mode side are obtained by the general limit cycle scheme of operating only the heater (in the first half of autotuning). The heater and cooler are then operated to generate a limit cycle (in the second half of autotuning). The proportional band of PID parameters on the cool mode side is calculated on the basis of a limit cycle amplitude in the first half of autotuning and a limit cycle amplitude in the second half of autotuning. Thereafter, autotuning is terminated.
Some apparatus using heat/cool control is designed such that forced heating by a heater is required to raise the temperature, and forced cooling by a cooler is required to lower the temperature. Some thermostatic test apparatus is used at room temperature (about 25° C.). In such an apparatus, autotuning is performed near room temperature. Assume that a limit cycle is to be generated by using only the heater at this time. In this case, after the intra-chamber temperature is raised, the temperature must be lowered while the apparatus is left standing at room temperature. As a consequence, a condition occurs in which the temperature hardly drops, i.e., a condition occurs in which it is impossible to generate a limit cycle. Likewise, assume that a limit cycle is to be generated by using only the cooler. In this case, after the intra-chamber temperature is lowered, the temperature must be raised while the apparatus is left standing at room temperature. As a consequence, a condition occurs in which the temperature hardly rises.
If a limit cycle cannot be generated by using only the heater or cooler, since it is difficult to generate a limit cycle by operating only the heater as in the above autotuning method, the conventional autotuning method cannot be used. Conventionally, as described above, in the heat/cool control system which cannot generate a limit cycle by using only the heater or cooler, it is impossible to obtain control parameters suitable for the heat mode and cool mode, respectively.
The above description is based on the premise that autotuning is performed near room temperature. This is, however, an example of autotuning. In some case, when a chamber is left standing after the intra-temperature is raised, the equilibrium point of temperature is settled at a temperature higher than room temperature due to a surrounding environment or the heat capacity of a furnace body. In such a case, in order to lower the temperature, forced cooling by a cooler is required. In contrast to this, in some case, when a chamber is left standing after the intra-temperature is lowered, the equilibrium point of temperature is settled at a low temperature. In such a case, in order to raise the temperature, forced hating by a heater is required. Therefore, when autotuning is to be performed at a temperature other than room temperature, the above problem can arise.